PBX systems are widely used, for example, for office telephone systems wherein the private exchange provides a number of users access to one or more outside telephone trunk lines. Telephones specifically adapted for use in a PBX system, hereinafter "PBX telephones", are often designed to provide such features as speed-dialing, conferencing, last number redial, call forwarding, and other desirable features that are not ordinarily provided by a standard telephone. In order to effectuate such features, a signaling channel is provided between each of the PBX telephones and the private exchange. In some PBX systems, the signaling channel is realized by a separate conductor pair that is connected in parallel with a standard tip-ring wire pair between the PBX telephone and the private exchange. In other PBX systems, the signaling channel is provided directly along the tip-ring wire pair by amplitude modulated tones that are outside of the voice band.
For example, in one known PBX system, signaling between the private exchange and each of the PBX telephones is provided at 8 kHz. In another PBX system, signaling is provided at 32 kHz. Such tones are easily distinguished from speech signals by the private exchange, since the standard telephone voice band is ordinarily located between about 300 Hz and about 3500 Hz.
There are a variety of signaling protocols between private exchanges and their associated PBX telephones which are often proprietary in nature. Users of such systems are ordinarily required to use only those PBX telephones that are specifically designed to be compatible with their selected PBX system. A distinct inconvenience of that requirement results from the fact that the conferencing, or hands-free, operation of PBX telephones usually relies upon a voice-switching circuit for preventing undesirable feedback between the loudspeaker and the microphone, which would otherwise result in howling or echo. Such voice-switching circuits introduce undesirable effects such as sentence clipping and an inability to conduct simultaneous two-way, or full-duplex, voice communication.
Digital echo cancellation has made it possible to construct conferencing telephones that can provide high quality, full-duplex voice communication. In a telephone employing digital echo cancellation, a transversal filter is connected between the incoming and outgoing signal paths. The filter performs a convolution of the incoming signal with the transfer function between the loudspeaker and the microphone. The resulting signal is subtracted from the outgoing signal so that the incoming signal as reproduced by the loudspeaker is substantially canceled from the outgoing signal produced by the microphone. One such full-duplex conference telephone is the CONFERENCEMASTER telephone, manufactured by Coherent Communications Systems Corporation of Hauppauge, New York, assignee of the present application.
Because of the manufacturing difficulties of producing many different models of full-duplex conference telephones that are each adapted to function compatibly with a particular proprietary PBX system, it would be desirable to provide an adapter that allows a user to connect a full-duplex conferencing telephone, having a standard tip-ring interface, in tandem with the user's PBX telephone. It would be desirable for such an adapter to maintain the signaling channel between the PBX telephone and the private exchange, while also effectively attenuating voice band signals between the PBX telephone and the private exchange. Voice-band attenuation would be desirable so that any voice signals picked up by the microphone of the PBX telephone do not interfere with the echo cancellation function provided by the full-duplex conferencing telephone. Additionally, it would be desirable to configure such a device, or the conferencing telephone, so that out-of-band signals (i.e., signals with frequencies outside the voice band) along the tip-ring connection between the PBX telephone and the private exchange are not reproduced by the loudspeaker of the conferencing telephone.
Interface devices for connecting standard telephones to PBX systems are described in U.S. Pat. Nos. 4,759,054 and 5,097,501. The interface devices described in those patents are designed to connect to PBX systems wherein signaling is conducted along a conductor pair that is separate from the tip-ring conductor pair. Hence, those devices do not provide for attenuating or blocking out-of-band signals between the standard telephones and the private exchange. Additionally, both of the systems described in the aforementioned patents allow simultaneous use of the PBX telephones and the standard telephones, and would therefore interfere with the echo cancellation function of a full-duplex conferencing telephone.
U.S. Pat. No. 4,088,844 describes an interface circuit for a tip-ring telephone, which provides the signaling function of a PBX telephone, so that the tip-ring telephone can be connected to a private exchange. In order to employ such interface circuitry to connect a full-duplex conference telephone to a PBX system, it would be necessary to produce such interface circuitry in a number of different configurations to provide compatibility with the variety of known PBX systems. Additionally, the user is then deprived of the numerous additional features provided by the PBX telephone.
U.S. Pat. No. 5,237,605 relates to a tip-ring telephone having an internal module that provides a separate PBX connection. As in the case of U.S. Pat. No. 4,088,884, the incorporation of such a module into a full-duplex conference telephone would require a variety of different configurations to correspond to the variety of known PBX systems.